In U.S. Pat. Nos. 4,275,385 and 5,103,108 infrared communication systems are described wherein a person or device carries a portable infrared transceiver with which signals can be relayed via ceiling located modules. This infrared communication technique has been extended so that a central locating facility can determine the location of the portable infrared transceiver, see for example U.S. Pat. No. 5,319,191. This is done by placing an infrared communicator in each room of an enclosure and providing suitable cabling from each room located module to a central control facility. The latter may then determine from the code signals from the portable infrared device where the person or apparatus is located.
A problem with such infrared communication system is that each of the room located infrared modules needs to be connected by cable to a central control. The cabling is expensive to install and available cabling is not always suitable for communication at the desired transmission rate. The cabling costs can constitute between 30 to 60 percent of the costs for an entire infrared communication system.
Another problem is found to arise in areas where a multiple of fixed IR units are used. In such a situation a portable device may be reported to be in more than one area at a time by different responding fixed IR units. To resolve such ambiguity, the infrared signal strength is measured and reported along with the identification codes to enable a central processor to determine the most likely location for the portable device. To enable a reliable determination of the location or when a portable device has left an area, an impracticably accurate (i.e. +/-1 db) logarithmic signal strength measurement is required. Such measurements impose expensive hardware requirements, which in the face of the high cabling costs for the wiring of the fixed IR units drive the price of an IR locator system too high for many applications.
When an IR locator system employs a regularly transmitting IR portable badge to a fixed IR receiving unit, frequent location updates, of the order of 20 times a minute, are needed to keep track of the IR portable device. Such updates determine the time before one can note that a person or apparatus bearing the badge has changed location. Since, several seconds for such determination can be too long, an increase in the transmission rate would be needed. In such case, however, the drain on the portable device's battery increases and its corresponding useful life or recharge interval is unacceptably reduced. One can increase the battery size, but this also represents an undesirable expense and an overly heavy badge.
RF devices for locating and monitoring people are described in U.S. Pat. Nos. 4,598,275; 4,814,751; 5,153,584; and 5,317,309. Such systems tend to propagate through walls of a room so that the precise room location of a person is not always available. Such RF systems are prone to interference from extraneous sources and from other users at the same frequencies.
These shortcomings of the prior art IR locator systems are avoided with an IR locator system in accordance with the invention.